Hi Leah,
I can relate to your beginner's questions, because I'm at the end of my first season of cycling. Knew nothing at the beginning, but learned by doing a summer charity ride similar to the MS150. Just traded in my first purchase for a much more suitable bike for me.

From what I can tell from reading this forum, as well as my own experience - it takes a certain amount of road cycling experience to figure out your riding needs and style. If you have an excellent relationship with a good bike shop, their advice can help you make the best possible choice of first bike. But there's still no substitute for learning through your own experience. Also, please keep in mind that there are many people out there who favor one material over another (steel, aluminum, carbon, titanium, etc.), and that may influence their advice to you. What most people seem to agree on is that proper fit to your body dimensions is most important - you can have the most expensive bike in the world, but if it doesn't fit, you won't be happy.

Because of the importance of very careful fit, there are a couple of ways you could go - pay a bike store for a fitting session, or if you're lucky, maybe you know someone who can give you expert advice. A cheaper (free) option is the one I took: take careful measurements of yourself, and use an online fit calculator as a ***starting point*** for narrowing down possible brands and models. (I repeat, this is just a starting point!!!) The calculator that I found to be very helpful and easy to use was at wrenchscience.com.

If you take those measurements very carefully (get someone to help you), you will find that certain brands/models will emerge as good candidates for you to consider first, because of their geometries. I think it's a reasonable way for a newbie to try narrowing down the field, and to come up with a reasonable list of bikes to test ride. Also, this is exactly how the expensive custom framebuilders start their process - by having the customer fill out a questionnaire that includes all of the key measurements.

I actually put enough faith in the calculator's output for me to buy my current bike on eBay, based only on its geometry and my current knowledge of my riding preferences, with no test ride. So far - very happy!

Given your budget, it might not be such a crazy idea to spend $100-200 on a fitting session, and then the rest of the money on getting the best possible used bike you can find that fits your dimensions. There are a lot of barely used bikes out there, because road cycling is definitely a sport that many people try and quit rather quickly. Also because many people get their size wrong the first time!

Good luck finding your first bike! --Jen